Inverters have long been used to convert DC power into AC power for one or more AC loads. An inverter may be of the pulse-width modulated (PWM) type having a pair of switches which are operated in accordance with a PWM pattern to produce output power having a substantial fundamental component and harmonics thereof. An inverter control detects an operational parameter, such as load magnitude, at a point of regulation (POR) and selects an appropriate PWM pattern based upon such detected parameter. In this way, the output of the inverter may be regulated.
Kirchberg, Jr., et al., U.S. Pat. No. 5,034,876, assigned to the assignee of the instant application and the disclosure of which is hereby incorporated by reference herein, discloses an inverter which is controlled by a PWM controller wherein the latter is responsive to the voltage magnitude and current magnitude at the POR. The PWM controller selects a provisional PWM pattern from a memory, models the response of the inverter and filter coupled to the output of the inverter based upon the assumption that the inverter is controlled in accordance with the provisional PWM pattern and checks to determine whether the inverter output developed from the modeled response contains harmonic content less than a specified magnitude. If so, the provisional PWM pattern is thereafter used to actually control the inverter. If the harmonic content in the modeled inverter output is greater than the specified magnitude, a different PWM pattern is provisionally selected and the response of the inverter and filter is again modeled, this time using the newly provisionally selected PWM pattern. If the modeled inverter output contains harmonic content less than the specified magnitude, then the newly provisionally selected PWM pattern is used to control the inverter switches. Otherwise, yet another PWM pattern is provisionally selected and the process is repeated until a pattern is found which results in the modeled inventer output having harmonic content less than the specified magnitude.
Inverters of the high-power type typically utilize series-connected pairs of high power switches which have substantial turnoff times. Typically, a fixed dwell interval is interposed between turn off of one switch and turn on of the other switch of the pair to prevent short circuiting of the DC link coupled to the inverter input. In such a system, the output voltage of the inverter can be indeterminate during the dwell interval. The resulting inability to accurately predict the actual point at which the output voltage switched between positive and negative voltage levels resulted in the inability to select an optimum PWM pattern for the inverter switches, in turn resulting in the inability to develop a PWM output having minimized harmonic content. As a result, the output filter coupled to the inverter had to be sized to account for this increased harmonic content, thereby undesirably increasing the size and weight of the overall inverter system. This increase in size and weight is particularly disadvantageous in applications where size and weight must be kept to a minimum, such as in aircraft.
Inverter controls are also disclosed in Kernick, et al., U.S. Pat. No. 3,648,150, Glennon, U.S. Pat. Nos. 4,382,275 and 4,527,226, Parro, II, U.S. Pat. No. 4,595,976, JaQuay, U.S. Pat. No. 4,626,979, Shekhawat, et al., U.S. Pat. No. 4,635,177, Sekino, et al., U.S. Pat. No. 4,646,221, Hancock, U.S. Pat. No. 4,757,432, Kawabata, et al., U.S. Pat. No. 4,757,434, Hanei, et al., U.S. Pat. No. 4,772,996, Muto, et al., U.S. Pat. No. 4,480,299 and Takahashi, U.S. Pat. No. 4,800,478.